1. Field of the Invention
The present invention relates to an optical recording medium for writing and reading information with light, particularly a phase change optical recording medium.
2. Description of the Related Art
The phase change optical recording medium utilizes, for recording information, a reversible structural change (phase change) between an amorphous state and a crystalline state of a material created by irradiation of light, particularly a laser beam. Such a phase change optical recording medium may have a high information processing rate and a high recording capacity.
As a result, a phase change optical recording medium with performance of a high speed erase and rewrite of written information in pratical use has been sought. To attain this, it is essential to have stable repeated overwrite operation. Overwriting means writing new information over previously written information while it is being erased. It is preferred that the possible number of times of erase and write is high.
A phase change optical recording medium is commercially available as a rewritable optical disc. For example, a 120 mm phase change optical recording disc is commercially available. A typical disc comprises a stack structure of a polycarbonate substrate, a first transparent dielectric layer of ZnS.SiO.sub.2, a phase change recording layer of GeSbTe, a second transparent dielectric layer of ZnS.SiO.sub.2, a reflection layer of an Al alloy and a UV-cured organic resin coated layer in this order.
The levels of erase and overwrite repetition durability of these commercially available discs are practically acceptable, but it is still desired that this repetition durability is further improved from the viewpoint of the reliability of the products. It is also desired that the erase and overwrite repetition durability is further improved from the viewpoint of the yield of production since the durability is largely influenced by the conditions of production such as sputtering conditions.
Further, since a high density overwrite disc which is now under development uses mark edge recording method, deterioration of the recording layer by repeated overwriting significantly adversely affects the quality of the signal and the overwrite repetition durability is important.
It is known that the repeating overwrite characteristics are influenced by various properties of the materials of a recording layer and a protecting layer. Thus, the following materials, for example, have been developed as the materials for a protecting layer: non-oxides such as an Al nitride, a Si nitride, MgF.sub.2 and AlF.sub.3, oxides such as SiO.sub.2, Al.sub.2 O.sub.3, ZrO.sub.2 and TiO.sub.2, chalcogenides such as ZnS, mixtures of a chalcogenide and an oxide such as a mixture of ZnS.SiO.sub.2, and the like. These materials are deposited as a thin layer by a known method and are used as protecting layers but the obtained discs do not allow a sufficient number of repetitions of overwriting.
The present invention aims to improve the above repetition durability.
The protecting layer of a phase change optical recording medium must be excellent in heat resistance and mechanical properties, since it is subjected to thermal and mechanical loads during write and erase, anid also must have a function of protecting the recording layer during storage of the optical medium. Further it must have a low thermal conductivity from the viewpoints of recording sensitivity and repetition durability.
The protecting layer of an oxide or nitride has a weak adhesive force with a recording layer comprising a chalcogen. As a result, during storage in a high temperature and humidity environment, the protecting layer may be peeled off or cracks may be formed in the protecting layer. Partly due to weakness in adhesive force between the protecting layer and the recording layer, heat supplied to the recording layer from a laser beam is not dissipated at an appropriate rate and the recording marks are connected with each other in which the material may flow in a direction, resulting in reduction in the number of erase and overwrite repetition.
Some methods have been proposed to improve the erase and overwrite repetition property. For example, Japanese Unexamined Patent Publication (Kokai) No. 6-139615 reported that provision of an adhesion promoting layer of Al.sub.2 O.sub.3, GeO.sub.2, SiO.sub.2, Ta.sub.2 O.sub.5, Y.sub.2 O.sub.3, or the like between a protecting layer and a reflection layer and/or between a protecting layer and a recording layer improves the number of erase and overwrite repetition. Japanese Unexamined Patent Publication (Kokai) No. 4-143937 reported that adoption of a ceramic protecting layer with high heat resistance and mechanical properties improved the repetition durability and provision of a ZnS anchoring layer on both sides of a recording layer improved the long Lime storage stability. Japanese Unexamined Patent Publication (Kokai) No. 7-307036 proposed a protecting layer with a dual layer structure comprising a second protecting layer of a low thermally conductive mixture of ZnS and an oxide in contact with the recording layer and a third protecting layer of a high Young's modulus in contact with the reflection layer.
It is supposed that these provide improvements to some extent. However, in accordance with the investigation by the inventors, the improvement in the number of erase and overwrite repetition is not sufficient.
In accordance with the investigation by the inventors, the following conditions must be met to have a sufficient erase and overwrite repetition durability: (1) The wettability between an adhesion promoting layer and a recording layer of a chalcogenide which is fused and made amorphous for write must be excellent, and the adhesion promoting layer must not be deteriorated by repeatedly exposed high temperature. (2) The layers must have a sufficient adhesion with each other. (It is supposed that the adhesion is low between crystalline layers due to misalignment of crystal lattice spacings and the adhesion may be rather higher between an amorphous layer and a crystalline layer. Here, the term "amorphous" means that, by the X ray diffraction, the diffraction spectrum does not have definite peaks while broad spectrums having a full width half maximum of about 5 degrees or more may exist.) (3) The adhesion of the adhesion promoting layer with the reflection layer must be excellent. In the prior art, only the adhesion of the adhesion promoting layer with the recording layer is considered but the adhesion of the adhesion promoting layer with the reflection layer is often not considered, which is one of the reasons why a sufficient erase and overwrite repetition durability was not obtained.
Also, since an excess laser power may deteriorate the repetition durability, the heat conduction coefficients of the adhesion promoting layer and the protecting layer must be sufficiently small, in order to have a sufficient recording sensitivity at a low laser power which is required for a practical disc medium.
From these requirements the above reports and proposes of the prior art may be considered as below:
Japanese Unexamined Patent Publication (Kokai) No. 6-139615 adopts an adhesion promoting layer of an oxide. It is considered that a chalcogen-containing recording layer has a low affinity with an oxide and therefore does not have a sufficient adhesion with the oxide adhesion promoting layer. Even if adhesion between an oxide adhesion promoting layer and a non-fused recording layer is excellent, the wettability between an oxide adhesion promoting layer and a recording layer fused during writing is low, which results in repellence and flow of the fused material of the recording layer and results in insufficient repetition durability of the disc. It is also supposed that one of the reasons for the insufficient repetition durability is a lack of consideration of the heat resistance of the protecting layer, which causes change in the crystallinity of the protecting layer by repeated heating and thus deterioration of the repetition durability.
Japanese Unexamined Patent Publication (Kokai) No. 4-143937 reported that a ceramic protecting layer with high mechanical properties improved the repetition durability. However, the ceramic protecting layer is considered not to have a sufficient adhesion with the reflection layer and the thermal conduction of the protecting layer is not considered. Further, an anchoring layer of ZnS with a thickness of only 50 nm may improve the long time storage stability but does not have an effect in improving the repetition durability due to a change in crystallinity by repeated heating.
Japanese Unexamined Patent Publication (Kokai) No. 7-307036 uses a protecting layer with a high Young's modulus in contact with a reflection layer, but the adhesion between the reflection layer and the protecting layer is not considered, and the wettability between the the protecting layer in contact with the recording layer and e recording layer is not considered to be sufficient.
A protecting layer of mixture of a chalcogenide such as ZnS and an oxide such as SiO.sub.2 has a low thermal conduction and improved the repetition durability to some extent (Japanese Examined Patent Publication (Kokoku) No. 4-74785). JPP'785 mentioned that a medium comprising a ZnS protecting layer has excellent initial characteristics but a poor erase and write repetition durability, and the reason for improvement of the durability by addition of SiO.sub.2 to ZnS is not clear but is considered that addition of SiO.sub.2 makes the layer amorphous and reduces the thermal conduction, which improves efficiency of temperature elevation of an optically active layer, or a recording layer, by a supplied laser power. According to JP'036, ZnS has an excellent initial adhesion with a recording layer of a chalcogen alloy such as GeSbTe, but has a poor durability since crystal grain growth is caused by repeated erase and write. It is described that addition of SiO.sub.2, makes the layer amorphous and improves the repetition durability.
However, according to the investigation by the present inventors, only a layer of ZnS.SiO.sub.2 does not provide a sufficient overwrite repetition durability. The reason is considered as below: Addition of SiO.sub.2 adversely affects the wettability with a chalcogen alloy recording layer as GeSbTe, which is not preferable for improvement in the repetition durability. That is, the amount of SiO.sub.2 added to a ZnS.SiO.sub.2 mixture layer must be determined on a delicate balance and the tolerance range thereof is narrow, which not only makes the productivity low but also is the reason why a sufficient repetition durability is not exhibited.
The object of the present invention is to solve the above problems and to provide a phase change optical recording medium with an improved adhesion between the protecting layer and the recording layer and without peeling of a layer, formation of cracks and flow of the recording layer.